Polymer type ferroelectric materials have advantages such as flexibility, light-weight, good processability and low price as compared with inorganic ferroelectric materials such as ceramics. There are known, as represented examples thereof, vinylidene fluoride polymers such as polyvinylidene fluoride (PVdF) and vinylidene fluoride/trifluoroethylene (VdF/TrFE) copolymer.
With respect to PVdF, crystal structures thereof are roughly classified into three kinds such as I-form (also said to be β-form), II-form (α-form) and III-form (γ-form). Among them, it is only I-form crystal that can sufficiently exhibit high ferroelectricity.
High molecular weight PVdF is prepared by a radical polymerization method forms II-form crystal structure and does not exhibit ferroelectricity as is. In order to convert II-form crystal structure of PVdF to I-form crystal structure, there are required complicated post-steps such as stretching and heat-treating of a film or rapid cooling under high pressure at casting.
Matsushige et al have studied formation of a thin film of vinylidene fluoride oligomer having I-form crystal structure by using vinylidene fluoride oligomer: CF3(CH2CF2)nI (number average degree of polymerization n=17) having II-form crystal structure (M & BE Vol. 11, No. 2, 145 (2000)).
However, Matsushige et al. have only studied vinylidene fluoride oligomers having an iodine atom at its end.
Okui et al. analyzed the crystal structure of vinylidene fluoride oligomer: CCl3(CH2CF2)nCl (number average degree of polymerization n=9) prepared by radical polymerization by using CCl4 as a chain transfer agent (telogen) and dinormalperoxy dicarbonate as a catalyst, and have reported that this oligomer was a mixture of crystal structures of I-form (β-form) and III-form (γ-form) and had a crystalline melting point Tm at two points (74° C. and 110° C.) (Polymer Journal, Vol. 30, No. 8, pp. 659 to 663 (1998), and POLYMER Vol. 38, No. 7, pp. 1677 to 1683 (1997)). However, Okui et al. have only studied vinylidene fluoride oligomers having an iodine atom at its end.
Other than these, there is a method of introducing a hydroxyl group into an end by a polymerization method using methanol as a chain transfer agent (telogen) (Macromol. Chem. Phys., 199, pp. 1271 to 1289 (1998)), however, only the polymerization method is studied, and a process for efficiently preparing a homopolymer having crystal structures of I-form (β-form) being capable of exhibiting ferroelectric characteristics at excellent purity is not indicated. Formation of a thin film certainly has not been studied at all.
In Macromolecules, 35, pp. 2682 to 2688 (2002), high molecular weight PVdF having a weight average molecular weight of 534,000 is dissolved in an organic solvent mixture of dimethylformaminde (DMF) with various solvents such as dimethyl sulfoxide (DMSO), acetonitrile, dimethylacetamide (DMA) and acetone, and influences of a dipole moment of an organic solvent and water content in formation of crystal structures of I-form (β-form) have been studied. However, crystal structures of only III-form (γ-form) or II-form (α-form) are obtained by precipitation from the organic solvent, and a PVdF film having crystal structures of I-form (β-form) at high purity has not been obtained (see Table 1 in Macromolecules, 35, pp. 2682 to 2688 (2002)).
JP-A-63-145353 discloses that a thin film mixture with improved adhesion to a substrate is obtained by forming a film by dissolving a ferroelectric fluorine-containing polymer such as polyvinylidene fluoride and a non-ferroelectric polymer into N-methyl-2-pyrrolidone with heating. However, spontaneous polarization of the ferroelectric thin film obtained in JP-A-63-145353 is as low as about 80 to 500 nC/cm2, thus, the thin film cannot be ever expected to be put into practical use.